True airspeed is perhaps the single most important parameter in assessing the performance of an aircraft whether in flight, landing, or taking off. The measurement of airspeed, even for the latest generation of aircraft, is performed using a Pitot-Static tube, also known as a Prandtl tube. In a Pitot-Static tube, the transducer measures the difference in pressure between static pressure and a total pressure by measuring the strain in a thin element using an electronic strain gauge. With measurements of air temperature and pressure to determine air density, the strain gauge measurement is converted to airspeed.
There are several known limitations to the Pitot-Static tube. First, if the velocity is low, the difference in pressures is very small and hard to accurately measure with the transducer. Accordingly, errors in the instrument could be greater than the measurement. This limitation means that Pitot-Static tubes do not work well near stall velocities, which is exactly the airspeed range where accurate measurements are imperative. Second, if the velocity is very high (supersonic), the assumptions of Bernoulli's equation have been violated and the measurement is wrong again. At the front of the tube, a shock wave appears that will change the total pressure. However, corrections for the shock wave can be applied to allow use of Pitot-Static tubes for high speed aircraft. Third, if the Pitot-Static tube becomes pinched through mechanical damage or clogged by dirt, ice, or even insects, the resulting pressures at the transducer will not represent the total and static pressures of the external flow. The transducer output that is then used to calculate a velocity will not represent the actual velocity of the flow.
Without a functioning Pitot-Static tube, a plane's flight computer will be unable to accurately and correctly determine speed, causing the automatic pilot control system to shut down and revert to manual control, forcing pilots to take control in sometimes challenging flying conditions. In some instances, issues with Pitot-Static tube have unfortunately been the cause of several accidents, such as Air France flight 447 and Birgenair flight 301, involving significant losses of life. There is a need for an improved system and method for measuring an airplane's airspeed that overcomes the above-mentioned drawbacks.